This invention relates to the pipe/cable locator-tracer art. In particular, a device for locating the position of a subterranean longitudinal metal object such as a pipe or cable is provided.
It has been known in the art to locate subterranean pipes or cables by first energizing these objects with a current which can be detected above ground with a search loop and appropriate receiver. Often, a transmitter is physically coupled to the receiver and the combination is used to induce a current in the object and detect the resulting field produced by the object. One of the problems encountered in the prior art in searching for subterranean objects according to these techniques is that the soil itself, particularly when mineralized, can distort the measurements obtained by a search loop and receiver operated above ground in connection with a transmitter. In particular, the signal received by the search loop can change with changes in height of the search loop above ground when there are mineralized conditions encountered.
Energizing the subterranean pipe by providing a transmitter with a large transmit coil above ground in the vicinity of a pipe has been known. The pipe below ground will provide a field which the search coil, located relatively close to the transmit coil and fixed in a preferred relationship with the transmit coil will receive. However, directly induced fields from the transmit coil which did not originate with the energized pipe will also be sensed by the search coil unless there are tedious mechanical adjustments made so that the search coil is maintained in a fixed relationship with the transmit coil. This solves the problem of direct coupling from transmitter coil to search coil, however, changes in the permeability of the soil which permits indirect coupling of the magnetic field associated with the transmitter coil with the search coil are not effectively dealt with by this technique. This effect results in a "ground" signal induced in the search loop in addition to a signal resulting from the energization of the pipe increasing the difficulty of detecting the pipe.
The sensitivity of these prior art devices is limited by the mechanical adjustments which are required to maintain the coils in a non-coupling relationship. Finer mechanical adjustments permit more sensitivity. Therefore, the operator of the locator is preoccupied with the maintaining of these adjustments.
In view of the foregoing difficulties, the present invention has been designed.